The invention relates to a tuning arrangement for a semiconductor diode laser with external resonator in a Littman arrangement which includes an optical transmission component, an optical refraction grid, a rotatable tuning arm having a resonator end mirror mounted thereon and control means for controlling the position of the tuning arm and the distance between the mirror plane and the axis of rotation of the tuning arm for compensating the chromatic dispersion of the components contained in the laser.
With a semiconductor diode operated in flow direction coherent light can be generated and emitted by means of stimulated emission. The wavelength of the emitted laser light is determined by the respective stoichiometry and the microscopic structure of the semiconductor laser material. Typical emission wavelengths are between 630 nm and 1550 nm.
For some semiconductor laser applications, it is necessary to install optical elements in the laser resonator for which an external resonator is used. The light emitted from a laser facet is collimated and is back-coupled into the semi-conductor laser by means of a separate (external) resonator end mirror. The laser facet directed toward the external resonator usually has an anti-reflection coating in order to provide for better coupling of the external resonator with the semiconductor laser.
With an external resonator which includes a wavelength-selective element such as an optical deflection grid, the emission wavelength can be adjusted over the amplification range of the laser. Typical band widths are between 12 nm and 120 nm depending on the use of semiconductor laser diodes with an emission wavelength of 630 nm or one with an emission wavelength of 1,550 nm.
Two typical arrangements for laser resonators which include wavelength-selective elements are the Littrow- and the Littman arrangement. A resonator is called a Littrow arrangement if it includes an optical deflection grid as a resonator end mirror wherein the directions of the incoming light and of the light which is dispersely reflected from the grid grooves and positively interferes, coincide. A folded resonator with deflection grid disposed between the resonator end mirrors is called a Littman arrangement. In this arrangement, the grid is positioned within the resonator in such a way that the first refraction order of the grid is on the resonator end mirror. The zero refraction order of the grid can then be used as operating beam of the laser. Consequently, the grid has a double function as a wavelength selective element and as an uncoupling element.
The Littman arrangement has the advantage that the illuminated grid area is larger, by a factor of 4 to 7, than that of the Littrow arrangement. As a result, the spectral selectivity of the grid is increased by the same factor so that, with a relatively large resonator length, the Littman arrangement guarantees single mode laser emissions and consequently achieves very small line widths. Another advantage is that commercially manufactured semiconductor lasers are often available only in housings which provide no access to the rear facet which is needed in the Littrow arrangement as an uncoupling mirror.
If during tuning of the emission wavelength of the laser system the resonator length is maintained constant, then the number m of nodal points of the standing light wave in the resonator changes what is termed a mode-hop. Consequently, the wavelength cannot be continuously tuned, but it leaps in discrete steps. As a result, it is difficult to tune in a desired wavelength and there may also be substantial fluctuations in the output power of the laser. Mode-hops can be avoided by varying the optical resonator length L.sub.opt during wavelength tuning in such a way that the wavelength .lambda..sub.R provided thereby is adjusted to the wavelength .lambda..sub.G determined by the grid. In terms of an equation, the condition: ##EQU1## must remain fulfilled. For mode-hop-free wavelength tuning of a laser which includes no dispersive media, it has been proposed to simply rotate a resonator mirror wherein the axis of rotation is disposed in the intersection of the mirror planes of the resonator end mirror and the plane of the refraction grid. However, semi-conductor lasers have a substantial chromatic dispersion because of their light amplification mechanisms. Consequently, the geometric length L.sub.geo differs from the optical length L.sub.opt. The relationship is as follows: EQU L.sub.opt =n(.lambda.).times.L.sub.geo ( 2)
wherein n(.lambda.) is the fraction index of the laser material whose value depends on the emission wavelength of the semiconductor laser. For a simplification of use the equation 2 is developed in a power series expansion to: ##EQU2## Mode-hop-free wavelength tuning of semiconductor lasers with external resonators is possible over large wavelength ranges only by taking the chromatic dispersion into consideration. A measure for the equalizing quality is the number i up to which the development coefficients n.sub.i can be taken into consideration. One speaks of "compensation of the dispersion of the j.sup.th order" when the terms n.sub.i including the term n.sub.j are exactly taken into consideration. A mode-hop-free wavelength tuning of semiconductor lasers with an external resonator over ranges larger than 60 nm to 80 nm is generally only possible if the dispersion of the 2.sup.nd order is taken exactly into consideration and the dispersion of the 3.sup.rd order is at least approximately taken into consideration.
An example for the compensation of the chromatic dispersion of all components of a semiconductor laser system-comprising semiconductor laser, a collimation lens system and an air-filled external resonator is described in a publication by Favre, LeGuen, "82 nm of continuous tunability for an external-cavity semiconductor laser", Electronics letters 27(2), pages 183-184, Jan. 17, 1991. Favre presents a semiconductor laser with a Littrow arrangement wherein, by means of an adjustment screw, the global chromatic dispersion of the semiconductor laser, of the imaging lens system and of the air in the external resonator can be compensated for, but only the dispersion of the first order. Furthermore, a mode-hop free wavelength tuning is not possible over the full amplification range of the laser; there are rather mode-hops which is the result of insufficient compensation for the contributions of the second and higher orders of the chromatic dispersion of the laser system.
Another example for the compensation of the chromatic dispersion of all the components of a semi-conductor laser system consisting of a semiconductor laser, a collimation lens system and an air-filled external resonator is described in WO94/08371. In this case, a semiconductor laser with a Littman arrangement is utilized wherein the axis of rotation of the mirror arm is so selected that the chromatic dispersion of the first order can be accurately compensated whereas for the dispersion of the second order a non-adjustable preset state is selected. As a result, a mode-hop free wavelength tuning over the full tuning range of the laser system is not possible.
It is the object of the present invention to provide a means for the generation of coherent light with continuous and rapidly tunable wavelength with small spectral line width. Particularly, a tuning arrangement for a semiconductor laser with external resonator is to be provided which facilitates mode-hop free wavelength tuning over the full amplification range of semiconductor lasers with a Littman arrangement.